This invention relates to rapidly initiating operation of a fuel cell powered electric vehicle, at sub-freezing temperature, by means of one or more of: providing excess reactant gas or cold reactant gas to the proton exchange membrane (PEM) fuel cell stack which powers the load, such as a vehicle propulsion system, connecting the load to the stack within 20 seconds of reactant gas flow or when open circuit voltage is detected, previously draining hydrophilic support plates, connecting coolant only after several minutes or when sufficient water has melted.
It is generally agreed that one difficulty with utilizing fuel cells to power the propulsion system of electric vehicles is the requirement that such vehicles be operable at temperatures below that at which water will freeze. Freezing provides potential mechanical damage as a consequence of the expansion of ice, and presents problems due to the inseparability of water and the fuel cell processes. Heretofore, various methods of initiating operation of a fuel cell, preparatory to the operation of an electric vehicle, have concentrated on providing heat, either by reaction or combustion of fuel, or by means of battery power, to various water and other coolant conduits and reservoirs. Other efforts are directed toward processes designed to accelerate the rate at which a fuel cell stack will heat up to above-freezing temperatures, as a consequence of its own operation. In U.S. Pat. No. 5,798,186, the fuel cell is warmed up simply by connecting a load across it while stochiometric fuel and oxidant are supplied to the stack. In one experiment, with the fuel, oxidant and coolant water passages all having been purged of water upon previous shutdown of the stack, application of hydrogen and air at room temperature caused a temperature in the core of a ten cell stack to advance from xe2x88x9211xc2x0 C. to 0xc2x0 C. in about one minute. A four cell stack, in which only the reactant channels (and not the coolant channel) were purged upon previous shut down, required five minutes, after circulation of hydrogen and oxygen began and a 50 amp load was connected, to increase from xe2x88x9219xc2x0 C. to 0xc2x0 C. Coolant was not circulated until about 23 minutes after startup. In a four cell stack in which none of the channels were purged at the prior shut down, flow of warm hydrogen did not begin to occur until after four minutes, and 12 minutes expired between startup at xe2x88x9223xc2x0 C. and reaching 0xc2x0 C. within the core of a four cell stack. In U.S. Pat. No. 6,329,089, individual fuel cells at xe2x88x925xc2x0 C. started with room temperature hydrogen and air reached 0.5 amps per cm2 in five minutes. With a short circuit load, a seven cell stack with a core temperature of xe2x88x9215xc2x0 C. reached 0.5 amps per cm2 nine minutes after prolonged short circuiting of the stack output. Performance of other experiments were less satisfactory.
For use in vehicles, such as automobiles, an electric propulsion system must be operating in less than one minute, preferably less than one-half minute, after initiating startup. None of the foregoing are capable of providing fuel cells operable in subfreezing temperatures, particularly as low as xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 F.).
Objects of the invention include: operating, at subfreezing temperature, an electric vehicle powered by a fuel cell within seconds of initiation; improved initiation of fuel cell powered, electric vehicle operation at subfreezing temperature; initiating fuel cell powered electric vehicle operation at subfreezing temperature with a minimal of waste power used for raising the temperature of apparatus and/or fluids; avoiding the need for heat exchangers and other apparatus to heat reactants or coolants above freezing; and avoiding use of battery power to start a fuel cell for powering a vehicle.
This invention is predicated on the discovery that the propulsion system of an electric vehicle powered by a PEM fuel cell can be powered from the fuel cell while the fuel cell stack is frozen. This invention is further predicated on the discovery that contrary to belief of the prior art, excess reactants, rather than reactant starvation, will permit extended operation of the fuel cell stack pending the ability to flow water through the stack. The invention is further predicated on the discovery that high flow of cold reactant gases through the reactant flow fields is not sufficient to cause freezing of product water, the heat generated in the membrane electrode assembly being sufficient, and sufficiently close to the reactant flow fields, to prevent freezing of product water or refreezing of melted water.
The invention is also predicated on the discovery that fuel cell operation without loss of performance or damage to the cells can be extended during a frozen startup by providing at least one of the reactant gases at a pressure in excess of the pressure of any water in the stack, which before operation of a water circulation system is typically atmospheric.
According to the present invention, a PEM fuel cell stack at subfreezing temperature is connected to a vehicle propulsion system or other electric load within a few seconds or as soon as the stack provides open circuit voltage. According to the invention, the fuel cell stack is started with more than a stochiometric flow of fuel and at least stochiometric flow, but preferably two-five times stochiometric flow of oxidant, which may be at subfreezing temperatures, or not, whereby to prolong operation without localized heating, thereby permitting the vehicle (or other load) to be used during the time that the apparatus and fluids are being heated to suitable, operational temperatures. The invention not only permits, but prefers operation with reactants which are at the same sub-freezing ambient temperature as the fuel cell stack itself, contrary to usage of the prior art, since this prolongs the onset of localized overheating.
In further accord with the invention, in systems in which porous water transport plates are used for water management, heating of the water stored as ice in the pores of the water transport plates, by heating up the mass of the stack as well as the water, the heat of fusion as the ice melts, and evaporative cooling of some of that water, further prolongs the period of time at which the vehicle can be operated with power from the fuel cell stack, without circulating coolant, before there is impermissible local heating within the fuel cell.
In accordance further with the invention, at least one of the reactant gases is provided to the fuel cell stack at a pressure of at least about 4 kPa (0.6 psi) above the pressure of any water in the water channels, which typically will be about atmospheric pressure. This prevents liquid water from pooling in the reactant channels, and flooding the electrode substrates, which is particularly important in the oxidant gas reactant channel where product water can accumulate.
Principal aspects of the present invention include starting an electric load, such as a vehicle or other load, the fuel cell stack of which is at subfreezing temperatures, before awaiting for the fuel cell stack to reach normal operating temperature, by supplying the fuel cell with at least twice stochiometric quantities of oxidant, and using substantially empty hydrophilic support plates for temporary product water storage thereby to allow the fuel cell to operate without circulating coolant until such time as all of the water systems are functional.